Semi-insulating gallium arsenide substrates have been previously used in the fabrication of a number of semiconductor devices. In addition to providing the "handle" necessary in batch processing of semiconductor devices, these substrates are frequently utilized as the means for supporting a plurality of adjacent components fabricated side by side in an epitaxial extension of the gallium arsenide substrate. This GaAs substrate must necessarily be a single crystal structure in cases where a GaAs epitaxial layer is formed thereon in the device fabrication process. Additionally, the semi-insulating resistivity of the GaAs substrate is typically on the order of 10.sup.7 - 10.sup.8 ohm. centimeters, and such high resistivities may be achieved by introducing chromium or oxygen into the GaAs melt from which the substrates are grown. Actually, GaAs substrates exhibiting a bulk resistivity anywhere within the range of 10.sup.6 - 10.sup.8 ohm. centimeters would be acceptable for use in the present process where the substrate is used as a common support and an electrical isolation medium for epitaxial GaAs devices.
GaAs semi-insulating substrates have been commercially available for many years. Since these substrates were, in the past, never directly doped to form active device regions, the specific amount of the chromium or oxygen dopant introduced in the GaAs melt was not considered to be particularly important. The dopant quantities of chromium or oxygen were required to be sufficient to raise the resistivity of the substrate thus produced from an undoped level on the order of 10.sup.14 carriers/cc to some level approaching the intrinsic resistivity of the GaAs, i.e., something on the order of 10.sup.8 or 10.sup.9 carriers/cc.
Because of the presence of either chromium or oxygen in semi-insulating GaAs substrates, and possibly because of the generally unspecified and normally unkown amount of such dopant in the GaAs crystal, it was generally felt by those skilled in the art that semiconductor devices of commercially acceptable quality could not be made by introducing impurities directly into the semi-insulating substrates. It was generally believed by workers in the art that the presence of chromium atoms, in the GaAs crystal for example, would unduly decrease carrier mobilities in the substrate and therefore would not permit the fabrication of commercially acceptable semiconductor devices therein. Chromium produces defect centers in the GaAs crystal which act as deep level traps in the GaAs band gap. These traps tend to unacceptably limit the carrier mobilities and to degrade the gain-versus-frequency characteristic of devices produced in the GaAs. This is true unless steps are taken to carefully limit the chromium or oxygen dopant levels to only those amounts necessary to produce a semi-insulating bulk resistivity on the order of 10.sup.6 - 10.sup.8 ohm. centimeters.
In the past, the generally accepted practice of making GaAs semiconductor devices and integrated circuits utilizing semi-insulating GaAs substrates was to epitaxially deposit a layer of lower resistivity GaAs material on the semi-insulating GaAs substrate and then to further treat this epitaxial layer in order to form active device regions. For example, in the fabrication of certain types of GaAs field-effect transistors, active device regions are formed in a GaAs epitaxial layer which is an extension of the semi-insulating GaAs substrate. Thus, in the fabrication of field effect transistor regions in the GaAs epitaxial layer, it didn't make any difference what the chromium doping levels in the underlying GaAs substrate were, so long as the substrate was semi-insulating to prevent undesirable current leakage between adjacent epitaxial islands or mesas.